Dozens of labs around the world have been experimenting with machines that can squirt out living cells, and thus build 3-D tissue structures, layer by layer. The latest research suggests the 3-D printing technique could be widely used with human embryonic stem cells. The findings were published in the journal Biofabrication.

Stem cells are typically extracted from a six-to-eight-cell embryo, which is about the size of the period at the end of this sentence. The cells are useful for bioengineering experiments because they can be turned into all other tissue-types in the body. A single stem-cell line can provide cells for years.

However, the cells are fragile and notoriously sensitive to human manipulation.

The new research shows “that we can print them without harming them, while maintaining their ability to differentiate,” said Wenmiao Shu, a bioengineer at Heriot-Watt University in Scotland and co-author of the study.

In the reported experiments, the embryonic stem cells were placed in two separate compartments. A computer program sent instructions that deposited the cells—in the form of spherical droplets—onto a plate. Each spheroid contained five to 140 cells. They “have controllable and repeatable sizes, and consequently they can be made to order for specific applications,” the study reported.

The experiments were done in collaboration with Scotland-based Roslin Cellab Ltd., which hopes to commercialize the technology.

The field of 3-D printing is making rapid progress in biology labs around the world. Scientists have printed a range of body parts, including heart valves, knee cartilage, bone implants, kidney cells, blood vessels and beating heart muscle. Commercial applications are years away.

Complex organs could prove a big challenge.

“We’ll need to generate the 3-D structures inside, such as the vascular structure, that provides nutrients and removes waste,” said Dr. Shu.